Magnetic recording medium and magnetic recording and reproducing apparatus

ABSTRACT

A thin film magnetic recording medium including a magnetic layer having a fluctuation field defined as S/X irr , where S is magnetic viscosity and X irr  is irreversible susceptibility X irr . The fluctuation field of the magnetic layer is not less than 15 oersteds.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This is a continuation of U.S. application Ser. No. 08/521,363,filed Aug. 31, 1995, the subject matter of which is incorporated byreference herein.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a magnetic recording mediumusing a ferromagnetic metal thin film, and more particularly to amagnetic recording medium having excellent electromagnetic transducingproperties, and a large capacity magnetic recording and reproducingapparatus.

[0003] For improving the recording density, increasing the output andreducing the noise of magnetic recording media, it is essential tomicronize magnetic particles in the case of a coated medium and crystalgrains in the case of a thin film medium. Regarding a medium using metalparticles that has heretofore been studied, for example, micronizationhas progressed and high-performance tapes such as Hi-8 (8-mmhigh-density magnetic tapes) using extra-fine particles having acylinder major axis length of approximately 200 nm and a cylinderdiameter of approximately 30 nm are now put to practical use.Incidentally, a plurality of particles are subjected to magneticreversal in a group and signals are recorded when magnetic particleshave been formed into a cluster agglomerate or when the interactionbetween crystal grains is strong even though the magnetic particles orcrystal grains of a magnetic medium are extremely fine. When theplurality of particles are subjected to magnetic reversal and when themagnetic reversal unit becomes larger, noise increases at the time ofreproducing data. In consequence, the density improvement is greatlyhampered.

[0004] The size of the magnetic reversal unit is relevant to magneticviscosity. In other words, it is considered that the greater thefluctuation field of magnetic viscosity becomes, the smaller themagnetic reversal unit is. A description has been given of a meaning ofthe fluctuation field of magnetic viscosity in the Journal of Physics F:Metal Physics, Vol. 14, pp L155 to L159 (1984). Further, a detaileddescription has also been given of the measurement conditions in theJournal of Magnetism and Magnetic Materials, Vol. 127, pp 233 to 240(1993). The principle of measuring the fluctuation field of magneticviscosity will subsequently be described.

[0005] When a new magnetic field is applied to a magnetic material, themagnetization I(t) often varies in relation to the logarithm lnt of thefield applied time:

I(t)=const.+S·lnt.  (1)

[0006] In this case, l(t) represents a magnetic moment per unit volume;and t, elapsed time after the new magnetic field is applied. Theviscosity coefficients has a positive value when the magnetic field isshifted in the positive direction and has a negative value when it isshifted in the negative direction. Moreover, it is known that S can beexpressed by the product of the irreversible susceptibility X_(irr) andthe fluctuation field H_(f). In other words, there is established.

S=X _(irr) ·H _(f)  (2)

[0007] Therefore, the fluctuation field is determined if S and X_(irr)are found experimentally. The fluctuation field is a quantityrepresenting the degree of the influence of thermal fluctuation, and agreater fluctuation field signifies that it is easily affected bythermal fluctuation and that the magnetic reversal unit is small insize.

[0008] The fluctuation field where the field strength is equal tocoercivity or remanence coercivity can also be found from the dependenceon the field applied time of the coercivity H_(c) or remanencecoercivity Hr. The coercivity or remanence coercivity, together withfield applied time t, often lowers in relation to

H _(c) (or H _(r))=−A·lnt+const.  (3)

[0009] as the application time elapses. All the specimens mentioned inthe present specification, satisfied the equation (3). When thecoercivity or remanence coercivity varies with the field applied time taccording to Eq. (3), it is known that A takes substantially the samevalue as that of the fluctuation field H_(f) where the field strength isequal to the coercivity or remanence coercivity. This procedure is notonly simple but also excellent in reproducibility. Hence, the value A istaken as the fluctuation field of magnetic viscosity according to thepresent invention.

[0010] By measurement at room temperature, the fluctuation field thusfound has the nature of becoming large in proportion to the absolutetemperature at the time of measurement. When a fluctuation field ismeasured at room temperatures ranging from 10° C. to 30° C. excluding25° C. (the absolute temperature: T) according to the present invention,the fluctuation field thus measured is multiplied by (298/T) to take theproduction as a fluctuation field H_(f) at 25° C.

[0011] In accordance with the conventional method, a Cr under-layer wasfirst formed on a mirror-polished disk made of Ni—P electroless-platedAl—Mg alloy, and then a CoCrTa magnetic layer together with a protectivecarbon film was formed thereon to fabricate a magnetic disk. The Crunder-layer, the magnetic layer and the protective layer were formed byAr-gas sputtering. In this case, the substrate temperature and the Arpressure were 300° C. and 2.0 milliTorr, respectively. Further, the Crunder-layer, the magnetic layer and the protective layer were 50 nm, 25nm and 10 nm thick, respectively. The composition of the CoCrTa magneticlayer is Co: 80%, Cr: 16%; Ta: 4%, expressed by atomic %. Thiscomposition will be expressed as CoCr₁₆Ta₄ The coercivity H_(c) and theremanence coercivity H_(r), were 1645 and 1655 oersteds, respectively.Further, the fluctuation fields of magnetic viscosity at 25° C. at thefield strength equal to the coercivity and at the field strength equalto the remanence coercivity were 13.5 and 13.2 oersteds, respectively.Thus the fluctuation fields of magnetic viscosity at 25° C. at the fieldstrength equal to the coercivity and at the field strength equal to theremanence coercivity exhibit substantially the same value: hereinafterthese are called simply the fluctuation field in this specification.

[0012] Incidentally, the measuring time of the fluctuation field rangedfrom 0 to 30 minutes.

[0013] A permalloy head having a gap length of 0.4 μm and a coil of 24turns was used to record magnetic data on the medium, and amagneto-resistive permalloy head was used to reproduce the data in orderto examine the electromagnetic transducing properties. The flying heightat the time of recording and reproducing data was 80 nm then. As aresult of measurement, noise at a longitudinal bit density of 150 kFCI(kilo Flux Change per Inch) was 22 μVrms.

[0014] Although a magnetic disk unit having a recording density of 300megabits/square inch could be fabricated by using this medium, amagnetic disk unit having a recording density of 1-gigabit/square inchcould not be fabricated.

[0015] An object of the present invention is to provide a magneticrecording medium and a magnetic recording and reproducing apparatussuitable for reducing noise at the time of reproducing data and forhigh-density recording.

SUMMARY OF THE INVENTION

[0016]FIG. 1 is an enlarged sectional view of a magnetic recordingmedium embodying the present invention. In FIG. 1, reference numeral 1denotes a nonmagnetic substrate of Ni—P-clad aluminum, Ni—P-cladaluminum-magnesium alloy, glass carbon or the like; 2, a nonmagneticunder-layer for controlling the crystal orientation and crystal grainsize of a magnetic film, which is a metallic layer of Cr, Cr—Mo, Cr—W,Cr—Ti, Cr—V or the like; 3, a ferromagnetic thin film of a cobalt-basedalloy such as Co—Cr—Ta, Co—Cr—Pt, Co—O, Co—Ni, Co—Cr, Co—Mo, Co—Ta,Co—Ni—Cr, Co—Ni—O or the like alloy; and 4, a protective lubricant layerin which a carbon film, an oxide film, a plasma polymerized film, fattyacid, perfluorocarbon carboxylic acid, perfluoropolyether or the likemay be used as a single or composite material. A ferromagnetic thin filmfor use as the magnetic layer 3 is desirably such that the fluctuationfield of magnetic viscosity at 25° C. at the field strength equal to theremanence coercivity or coercivity is not less than 15 oersteds, thecoercivity is not less than 2000 oersteds, and the thickness of themagnetic layer 3 is not less than 5 nm and not more than 30 nm. It ismore desirable that the fluctuation field of magnetic viscosity at 25°C. act the field strength equal to the remanence coercivity orcoercivity is not less than 20 oersteds. The ferromagnetic thin film isdesirably a cobalt-based ferromagnetic thin film containing at least onekind selected from a group consisting of Cr, Ta, Pt, Ni, Mo, V, Ti, Zr,Hf, Si, W and O, for example, a thin film containing cobalt of Co—Cr—Ta,Co—Cr—Pt, CO—O, Co—Ni, Co—Cr, Co—Mo, Co—Ta, Co—Ni—Cr, CO—NI—O or thelike.

[0017] A specific method for measuring the fluctuation field is asfollows:

[0018] In order to obtain a fluctuation field A, a magnetic field of−10,000 oersteds is applied to a specimen 7 mm square cut out of amagnetic disk before being subjected to dc-erase. Subsequently, apositive magnetic field slightly lower than the coercivity or remanencecoercivity is applied to the specimen to obtain time t until themagnetization or remanent magnetization decreases to zero. While thepositive magnetic field applied after the dc-erase is lowered gradually,the operation above is repeated. The fluctuation field A is found fromthe dependence of the coercivity or remanence coercivity on the fieldapplied time thus determined according to Eq. (3). The fluctuation fieldfound from the dependence of the coercivity on the field applied timeshows substantially the same value as that of the fluctuation fieldfound from the dependence of the remanence coercivity on the fieldapplied time. Because of measurement simplicity, the fluctuation field Awas found from the dependence of the remanence coercivity on the fieldapplied time according to the present invention. A vibrating samplemagnetometer of DMS (Digital Measurement Systems) Co. was employed forthe measurement purposes. The measuring temperature was at 25° C. andthe field applied time after the dc-erase was in a range of 0 to 30minutes then.

[0019] In a region of a short time less than several seconds, data from8 seconds up to 30 minutes was used when the fluctuation field was foundsince an error in the applied time tends to become greater.

[0020] Although the magnetic disk was an object in the example above,the present invention is also effective for magnetic recording mediasuch as magnetic tapes.

[0021] When a ferromagnetic thin film whose fluctuation field ofmagnetic viscosity at 25° C. at the field strength equal to theremanence coercivity or coercivity is not less than oersteds and whosecoercivity is not less than 2000 oersteds is used, and a magnetic layer3 whose thickness is not more than 5 nm and not less than 30 nm is used,it is possible to lower the noise level and to raise the S/N since thecluster size can be decreased at the time of magnetic reversal.

[0022] By the combination with a magnetic head using a metal magneticfilm in part of the magnetic pole, the medium capable of fast recordingis allowed to demonstrate its performance, so that a large-capacityrecording and reproducing apparatus can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a sectional view of a magnetic recording and reproducingapparatus embodying the present invention.

[0024]FIG. 2 is a characteristic diagram showing the relations betweenthe fluctuation field and the coercivity and between the fluctuationfield and the noise.

[0025]FIG. 3 is a characteristic diagram showing the relation betweenthe fluctuation field and the coercivity and between the fluctuationfield and the noise.

[0026]FIG. 4 is a characteristic diagram showing the relation betweenthe fluctuation field and the coercivity and between the fluctuationfield and the noise.

[0027]FIG. 5 is a sectional structural view of a magnetic disk unit.

DETAILED DESCRIPTION OF THE PREPARED EMBODIMENTS

[0028] Referring to an embodiment of the present invention, a detaileddescription will subsequently be given of the contents thereof.

[0029] [Embodiment 1]

[0030] A Cr-alloy under-layer was first formed on a mirror-polished diskmade of Ni—P electroless-plated Al—Mg alloy, and then a CoCrTa magneticlayer together with a protective carbon film was formed thereon tofabricate a magnetic disk.

[0031] The Cr-alloy under-layer, the magnetic layer and the protectivelayer were formed by Ar-gas sputtering. In this case, the Ar pressurewas 2.0 milliTorr. Cr—V, Cr—W, Cr—Ti, Cr—Si and Cr—Mo were used forCr-alloy under-layers to prepare 20 specimens in total different inunder-layer composition. The Cr-alloy layer, the magnetic layer and theprotective layer were 50 nm, 25 nm and 10 nm thick, respectively. Thecomposition of the CoCrTa magnetic layer thus utilized was CoCr₁₆Ta₄.The substrate temperature at the time of forming the Cr-alloyunder-layer and the protective carbon film was 300° C., whereas thesubstrate temperature at the time of forming the magnetic layer was250-300° C.

[0032] The coercivities H_(c) of the media thus prepared weredistributed in a range of 1500-2400 oersteds. The fluctuation fieldswere distributed in a range of 11.3-16.5 oersteds.

[0033] A permalloy head having a gap length of 0.4 μm and a coil of 24turns was used to record magnetic data on the media, and amagneto-resistive permalloy head was used to reproduce the data in orderto examine the electromagnetic transducing properties. The flying heightat the time of recording and reproducing data was 80 nm then. As aresult of measurement, the noise values at the longitudinal bit densityof 150 kFCI ranged from 18 to 25 μVrms. Table 1 collectively shows themeasurement results. TABLE 1 Composition Thickness of magnetic ofmagnetic Fluctuation Coercivity Noise film film (nm) field (Oe) (Oe)(μVrms) CoCr₁₆Ta₄ 25 11.3 1500 25.0 CoCr₁₆Ta₄ 25 11.5 1601 24.6CoCr₁₅Ta₄ 25 11.8 1685 24.3 CoCr₁₅Ta₄ 25 12.1 1723 24.5 CoCr₁₅Ta₄ 2512.3 1756 23.6 CoCr₁₆Ta₄ 25 12.6 1832 23.2 CoCr₁₆Ta₄ 25 12.9 1889 22.6CoCr₁₆Ta₄ 25 13.0 1890 22.5 CoCr₁₆Ta₄ 25 13.1 1926 22.0 CoCr₁₆Ta₄ 2513.2 1956 22.1 CoCr₁₆Ta₄ 25 13.4 1985 21.8 CoCr₁₆Ta₄ 25 13.6 1989 21.5CoCr₁₆Ta₄ 25 13.9 2023 21.3 CoCr₁₆Ta₄ 25 14.1 2056 21.4 CoCr₁₆Ta₄ 2514.3 2122 20.7 CoCr₁₆Ta₄ 25 14.6 2146 20.2 CoCr₁₆Ta₄ 25 14.7 2250 20.5CoCr₁₆Ta₄ 25 15.0 2280 19.5 CoCr₁₆Ta₄ 25 15.5 2420 19.0 CoCr₁₆Ta₄ 2516.5 2400 18.0

[0034]FIG. 2 shows the relations between the fluctuation field and thecoercivity and between the fluctuation field and the noise. As isobvious from FIG. 2, the noise values of the media whose fluctuationfields are of great values are low. The S/N values of the media ofhaving fluctuation fields of not less than 15 oersteds are higher thanthose of conventional media. it is therefore possible to make therecording density higher than conventional. The use of media withfluctuation fields of not less than 15.0 oersteds makes it possible tomanufacture magnetic disk units having a recording density of1-gigabit/square inch.

[0035] [Embodiment 2]

[0036] As in the first embodiment of the present invention, a Crunder-layer was first formed on a mirror-polished disk made of a Ni—Pelectroless-plated Al—Mg alloy, and then a CoCrPt magnetic layertogether with a protective carbon film was formed thereon to prepare amagnetic disk.

[0037] The Cr under-layer, the magnetic layer and the protective layerwere formed by Ar-gas sputtering. In this case, the Ar pressures was 2.0milliTorr. By varying the Cr content of the CoCrPt magnetic layer, 20specimens in total having compositions ranging from CoCr₁₅Pt₈ toCoCr₂₃Pt₈ were fabricated. The Cr under-layer, the magnetic layer andthe protective layer were 50 nm, 25 nm and 10 nm thick, respectively.The substrate temperature at the time of forming the Cr under-layer, themagnetic layer and the protective carbon film was 300° C. The coercivityH_(c) of the media thus fabricated were distributed in a range of1800-2800 oersteds. The fluctuation fields were distributed in a rangeof 12.0-20.5 oersteds.

[0038] As in the first embodiment of the present invention, theelectromagnetic transducing properties were measured. As a result thenoise values at the longitudinal bit density of 150 kFCI ranged from17.9 to 30 μVrms. Table 2 collectively shows the measurement results.TABLE 2 Composition Thickness of magnetic of magnetic FluctuationCoercivity Noise film film (nm) field (Oe) (Oe) (μVrms) CoCr₁₅Pt₈ 2512.0 1800 30.0 CoCr₁₅Pt₈ 25 12.6 1890 24.3 CoCr₁₆Pt₈ 25 12.2 1820 29.8CoCr₁₆Pt₈ 25 12.9 1850 26.3 CoCr₁₇Pt₈ 25 13.1 1920 23.6 CoCr₁₇Pt₈ 2513.3 2010 23.2 CoCr₁₇Pt₈ 25 13.7 1860 22.6 CoCr₁₈Pt₈ 25 13.3 2011 23.3CoCr₁₈Pt₈ 25 14.5 2306 22.1 CoCr₁₉Pt₈ 25 14.1 2215 22.2 CoCr₁₉Pt₈ 2514.6 2526 21.8 CoCr₂₀Pt₈ 25 14.9 2756 21.6 CoCr₂₀Pt₈ 25 15.0 2654 19.5CoCr₂₁Pt₈ 25 15.1 2345 18.8 CoCr₂₁Pt₈ 25 16.2 2645 18.8 CoCr₂₂Pt₈ 2515.3 2689 19.3 CoCr₂₂Pt₈ 25 16.8 2608 19.2 CoCr₂₃Pt₈ 25 17.5 2720 18.3CoCr₂₃Pt₈ 25 18.8 2800 19.1 CoCr₂₃Pt₈ 25 20.5 2750 17.9

[0039]FIG. 3 shows the relations between the fluctuation field and thecoercivity and between the fluctuation field and the noise. As isobvious form FIG. 3. the noise values of the media whose fluctuationfields have great values are conversely low as in the first embodimentof the present invention. The S/N value of media having fluctuationfields of not less than 15 oersteds are higher than those ofconventional ones. The use of media having fluctuation fields of notless than 15.0 oersteds enabled the manufacture of magnetic disk unitshaving a recording density of 1-gigabit/square inch. Moreover, the useof media having fluctuation fields of 20.5 oersteds and coercivity of2750 oersteds also enabled the manufacture of magnetic disk units havinga recording density of 1.5-gigabits/square inch. However, any one of themedia illustrated in this embodiment was unsuitable for producingmagnetic disks having a recording density of 2-gigabits/square inch.

[0040] [Embodiment 3]

[0041] A Cr under-layer was first formed on a mirror-polished glassdisk, and then a CoCrPt magnetic layer together with a protective carbonfilm was formed thereon to prepare a magnetic disk.

[0042] The Cr under-layer, the magnetic layer and the protective layerwere formed by Ar-gas sputtering. In this case, the Ar pressure was 2.0milliTorr, and the composition of the CoCrPt magnetic layer utilized wasCoCr₁₉Pt₈. Then 30 specimens were fabricated by varying the thickness ofthe Cr under-layer from 3 up to 50 nm, varying the thickness of themagnetic layers from 3 up to 30 nm and setting those of the protectivelayer to 10 nm. The substrate temperature at the time of forming the Crunder-layer, the magnetic layer and the protective carbon film was 300°C.

[0043] The coercivities H_(c) of the media thus fabricated weredistributed in a range of 1200-2900 oersteds. The fluctuation fieldswere distributed in a range of 11.2-68.3 oersteds.

[0044] As in the first embodiment of the present invention, theelectromagnetic transducing properties were measured. As a result, thenoise values at the longitudinal bit density of 150 kFCI widely rangedfrom 8 to 31 μVrms. Table 3 collectively shows the measurement results.TABLE 3 Composition Thickness of magnetic of magnetic FluctuationCoercivity Noise film film (nm) field (Oe) (Oe) (μVrms) CoCr₁₉Pt₈ 3011.2 1200 31.0 CoCr₁₉Pt₈ 30 11.7 1321 29.6 CoCr₁₉Pt₈ 30 15.1 2140 19.6CoCr₁₉Pt₈ 30 15.5 2206 21.9 CoCr₁₉Pt₈ 30 15.6 2518 21.9 CoCr₁₉Pt₈ 2712.1 1880 23.1 CoCr₁₉Pt₈ 27 13.2 1625 24.0 CoCr₁₉Pt₈ 27 13.8 1979 22.8CoCr₁₉Pt₈ 27 14.2 1959 21.5 CoCr₁₉Pt₈ 27 19.7 2356 18.2 CoCr₁₉Pt₈ 2512.4 1754 25.8 CoCr₁₉Pt₈ 25 15.3 1818 20.7 CoCr₁₉Pt₈ 25 16.5 2623 18.6CoCr₁₉Pt₈ 22 22.7 2218 18.8 CoCr₁₉Pt₈ 22 33.2 2756 16.7 CoCr₁₉Pt₈ 2023.6 2706 19.2 COCr₁₉Pt₈ 20 25.8 2900 18.3 CoCr₁₉Pt₈ 15 26.4 2800 17.8CoCr₁₉Pt₈ 15 29.4 2300 17.6 CoCr₁₉Pt₈ 15 38.7 2356 15.6 CoCr₁₉Pt₈ 1221.5 1957 19.3 CoCr₁₉Pt₈ 10 39.6 2408 14.5 CoCr₁₉Pt₈ 10 44.3 2036 13.3CoCr₁₉Pt₈ 10 58.6 1789 14.2 CoCr₁₉Pt₈ 10 53.2 2013 11.3 CoCr₁₉Pt₈ 8 60.31802 10.3 CoCr₁₉Pt₈ 8 67.5 1400 9.5 CoCr₁₉Pt₈ 5 61.2 1830 8.9 CoCr₁₉Pt₈5 63.5 1750 8.3 CoCr₁₉Pt₈ 3 68.3 1540 8.0

[0045]FIG. 4 shows the relation between the fluctuation field and thecoercivity and between the fluctuation field and the noise. As isobvious from FIG. 4, the noise values of the media whose fluctuationfields have great values are conversely low as in the first and secondembodiments of the present invention. The use of media whose thicknessesof the magnetic films were 10-27 nm, whose fluctuation fields were notless than 15 oersteds and whose coercivities were not less than 2000oersteds permitted the manufacture of magnetic disk units having arecording density of 1-gigabit/square inch. Moreover, the use of mediawhose thicknesses of the magnetic films were 10-25 nm thick, whosefluctuation fields were not less than 20 oersteds and whose coercivitieswere not less than 2000 oersteds also permitted the manufacture ofmagnetic disk units having a recording density of 1.5-gigabits/squareinch. Further, the use of media whose thicknesses of magnetic films were10-22 nm, whose fluctuation fields were not less than 30 oersteds andwhose coercivities were not less than 2000 oersteds permitted themanufacture of magnetic disk units having a recording density of2-gigabits/square inch. In this embodiment, media whose coercivitieswere not less than 2000 oersteds could not be fabricated when thefluctuation fields exceeded 60 oersteds. The outputs of the media whosecoercivities were less than 2000 oersteds were low and besides eventhough the noise values were low, it was impossible to manufacturemagnetic disk units having a recording density of not less than1-gigabit/square inch. If, however, a medium having a coercivity of notless than 2000 or 3000 oersteds is produced even though the fluctuationfield exceeds 60 oersteds, a magnetic disk unit having a recordingdensity of 2-gigabits or greater may be manufactured. Notwithstanding,the influence of thermal fluctuation will become critical if thefluctuation field exceeds {fraction (1/20)} of the coercivity, thusmaking the medium practically unusable. Although noise values were lowin the case of media whose magnetic films were less than 5 nm thick,sufficient outputs were not achieved, and consequently a magnetic diskunit having a recording density of not less than 1-gigabit/square inchcould not be produced using such media. If, further, the thickness ofthe magnetic film exceeds 30 nm, demagnetization in recording due to thethick film was too great. As a result, no magnetic disk unit having arecording density of 1-gigabit/square inch was produced.

[0046] [Embodiment 4]

[0047]FIG. 5 is a sectional structural view of a magnetic disk unitmanufactured by using media according to the present invention. In FIG.5, reference numeral 5 denotes magnetic recording media; 6, a magneticrecording medium drive; 7, a magnetic head; 8, a magnetic head drive;and 9, a recording and reproducing signal processor system. The use ofmagnetic recording media in the first to third embodiments of thepresent invention makes it possible to realize a recording density ofnot less than 1-gigabit/square inch.

[0048] As set forth above, according to the present invention, if aferromagnetic thin film whose fluctuation field of magnetic viscosity at25° C. at the field strength equal to the remanence coercivity orcoercivity is not less than 15 oersteds and whose coercivity thereof isnot less than 2000 oersteds is used, the S/N of the media can beremarkably improved, thus enabling high-density recording.

What is claimed is:
 1. A thin film magnetic recording medium comprisinga magnetic layer having a fluctuation field defined as S/X_(irr), whereS is magnetic viscosity and X_(irr) is irreversible susceptibilityX_(irr), the fluctuation field of the magnetic layer is not less than 15oersteds.
 2. A thin film magnetic recording medium according to theclaim 1 , wherein the fluctuation field of the magnetic layer is notless than 20 oersteds.
 3. A thin film magnetic recording mediumaccording to claim 1 , wherein the fluctuation field of the magneticlayer is not less than 30 oersteds.
 4. A thin film magnetic recordingmedium comprising a magnetic layer having a fluctuation field A which isnot less than 15 oersteds, wherein the fluctuation field A is determinedfrom the equation Hc=−A·lnt+const expressing the time dependence ofmagnetic coercivity Hc at a DC erased state against an external magneticfield and const is a mathematical constant.
 5. A thin film magneticrecording medium according to claim 4 , wherein the fluctuation field Aof the magnetic layer is not less than 20 oersteds.
 6. A thin filmmagnetic recording medium according to claim 4 , wherein the fluctuationfield of the magnetic layer is not less than 30 oersteds.
 7. A thin filmmagnetic recording medium comprising a magnetic layer having afluctuation field A which is not less than 15 oersteds, wherein thefluctuation field A is determined from the equation Hr=−A·lnt+constexpressing the time dependence of the remanence coercivity Hr at a DCerased state against an external magnetic field and const is amathematical constant.
 8. A thin film magnetic recording mediumaccording to the claim 7 , wherein the fluctuation field A of themagnetic layer is not less than 20 oersteds.
 9. A thin film magneticrecording medium according to the claim 7 , wherein the fluctuationfield A of the magnetic layer is not less than 30 oersteds.